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Related Experiment Videos

Off-axis primary-dose measurements using a mini-phantom

S A Johnsson1, C P Ceberg

  • 1Department of Radiation Physics, Lund University Hospital, Sweden.

Medical Physics
|May 1, 1997
PubMed
Summary
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Measurements of the primary off-axis ratio using a mini-phantom are accurate. Attenuation and scatter effects in the mini-phantom nearly cancel, eliminating the need for corrections in photon beam characterization.

Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Radiotherapy Physics

Background:

  • Photon beam characterization is crucial for radiotherapy accuracy.
  • Standard measurements use build-up caps, but mini-phantoms are recommended to avoid electron contamination.
  • Mini-phantoms introduce attenuation and scatter effects that can influence off-axis measurements.

Purpose of the Study:

  • To quantify the impact of attenuation and phantom scatter on primary off-axis ratio measurements.
  • To determine if corrections are necessary when using a mini-phantom for off-axis measurements.
  • To validate the use of mini-phantoms in photon beam characterization.

Main Methods:

  • Characterized off-axis beam-softening using narrow-beam transmission measurements.

Related Experiment Videos

  • Determined effective attenuation coefficients as a function of off-axis position.
  • Employed a semi-analytical phantom-scatter calculation dependent on the attenuation coefficient.

    • Main Results:

    • Developed a formalism for relative in-air measurements using mini-phantoms.
    • Demonstrated that attenuation and scatter effects in a mini-phantom nearly cancel (<1%) within 12.5 cm of the central axis.
    • Confirmed consistency for 6 and 18 MV photon beams.

    Conclusions:

    • Mini-phantom measurements for primary off-axis ratio are reliable.
    • No corrections for attenuation and scatter are required within 12.5 cm when using a mini-phantom.
    • This simplifies photon beam characterization in radiotherapy.